The hypothesis to be explored in Project #3 of this Program Project is that cytochrome P450 (P450) arachidonic acid (AA) metabolites provide a major signaling system for the extracellular calcium/polyvalent cation- sensing G protein-coupled receptor (CaSR) in the mammalian kidney. Genetic and physiological studies in man and laboratory animals have demonstrated that the CaSR is crucial not only for the extracellular calcium-dependent regulation of parathyroid hormone form parathyroid glands, but also for normal divalent mineral handling by the kidney. The CaSR is expressed in many epithelial segments of the mammalian kidney nephron, particularly at the urinary face of cells forming the initial proximal tubule and at the blood-interstitial face of the ascending segment of the loop of Henle called the thick ascending limb (TAL). Since salt transport processes in both the proximal tubule and TAL are involved in regulated divalent mineral ion excretion, activation of the CaSR in these nephron segments is expected to modulate calcium and magnesium loss in the urine. Modulation of transport processes by the CaSR in these segments also alter the kidney excretion of salt and water, processes that help to ensure that calcium and magnesium can be excreted with a reduced likelihood for kidney stone formation or nephrocalcinosis. Specifically, calcium activation of the CaSR in the TAL functions as in endogenous "loop" diuretic, and this mechanism may account for the beneficial effect of dietary calcium intake on blood pressure seen in laboratory animals and man. Using a combination of molecular, biochemical and physiological approaches, we will: a) identify the specific P450 gene(s) involved in CaSR signaling in the kidney tubule epithelial cells; b) define the specific P450 omega/omega-1 AA metabolites generated by activation of the CaSR; c) identify the enzymatic components of the CaSR-P450 signaling pathway; and d) define the physiological effects of CaSR action on salt transport process. Results of these studies should define roles for the CaSR in the renal regulation of divalent mineral homeostasis and salt and water balance.